This invention relates to a steel cord to be used for reinforcement of rubber products such as automobile tires, conveyors belts, etc.
Generally, this kind of steel cord is constructed by twisting a plural number of material wires and such steel cord is used as reinforcing material of automobile tires after being covered with a rubber compound in the state where a plural number of cords are arranged in parallel at prescribed intervals.
By the way, essential conditions required of this kind of steel cord are (1) to have excellent mechanical strength, (2) to be in good contact with rubber compound chemically and physically, and (3) to ensure good penetration of rubber compound to the inside of the steel cord.
The conventional steel cords 9 are of the so-called closed construction (1.times.2 to 1.times.6 construction) in which a plural number of material wires are intertwisted in close contact with one another as shown in FIGS. 6a-6e. Of those steel cords, each of the steel cords 9 indicated in FIGS. 6b-6e has a closed cavity D at the center of the cord formed by a plural number of material wires 10, 10 . . . constituting the steel cord. For that reason, if you pressurize and vulcanize a plural number of steel cords by pinching them between two rubber sheets, the rubber compound simply covers the periphery of the cords but does not penetrate into the cavity D. As a result, the complex sheet formed will not become a complete complex with the rubber (i.e. will not be fully integrated with the rubber) because a continuous cavity D exists longitudinally at the central part of the cord.
Moreover, the steel cord 9 of 1.times.2 construction indicated in FIG. 6a fails to become a complete complex with the rubber in the same way as the steel cord of 1.times.3 to 1.times.6 construction because the two material wires are inclose contact with each other continuously in the longitudinal direction preventing penetration of rubber compound in this part of close contact.
For that reason, in rubber products using the conventional type of steel cord 9 such as automobile tires, for example, the water content, produced as a result of condensation of gas emitted by the rubber compound or as a result of water which penetrated through cuts in the tire, will pass through the cavity in the longitudinal direction at the center of the cord, corroding the steel cord and sharply reducing its mechanical strength. Moreover, the tire is subject to the so-called "separation phenomenon" in which the rubber compound and the steel cord are separated from each other as a result of a drop in adhesion between the steel cord and the rubber compound. Furthermore, if repetitive bending stress is exerted on the rubber product having the steel cord 9, the steel cord will produce fretting wear to remarkably reduce its fatigue resistance, because the respective material wires 10, 10 . . . adjacent to one another are in linear contact with one another and the rubber compound hardly penetrates between those material wires.
Under such circumstances, a proposal was made in recent years to provide a steel cord 11 of so-called open construction (1.times.2 to 1.times.6 construction) in which the respective material wires 12, 12 . . . are intertwisted such that a clearance C is provided among them while arranging the respective material wires in a larger space as shown in FIGS. 7a-7e (for example, see JP 55-90692, which is exemplified in FIGS. 7b-7d).
To ensure sufficient penetration of rubber compound also in the central part of the cord in the open twisted cord 11, the clearance C between the respective material wires must be at least equal to 0.01 mm.
However, if the clearance C is set sufficiently large, the steel cord is beset by the following problems:
Namely, the steel cord provides a large free space for movement of material wires because the material wires adjacent to one another are in contact only about once in one twist pitch. Consequently, in the case of a steel cord 11 as shown in FIG. 7d, for example, an offset of the material wires is produced to make the twist uneven in the longitudinal direction as shown in FIGS. 8a-8b, and the steel cord is liable to buckle when repetitive bending stress acts on it. Moreover, the steel cord 11 was difficult to handle because of a large elongation of the cord under a low load and because the rubber compound did not penetrate to the inside of the cord sufficiently well because the clearance C decreased under the influence of the tension at low loads (5 kg, also applicable hereinafter) applied at the time of forming of the complex sheet.